EP1146767A2 - Méthode et dispositif pour minimiser le cas le plus défavorable de délai de mise en file d'attente dans un réseau commuté de communications avec des contraintes de transmission - Google Patents

Méthode et dispositif pour minimiser le cas le plus défavorable de délai de mise en file d'attente dans un réseau commuté de communications avec des contraintes de transmission Download PDF

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Publication number
EP1146767A2
EP1146767A2 EP01109085A EP01109085A EP1146767A2 EP 1146767 A2 EP1146767 A2 EP 1146767A2 EP 01109085 A EP01109085 A EP 01109085A EP 01109085 A EP01109085 A EP 01109085A EP 1146767 A2 EP1146767 A2 EP 1146767A2
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EP
European Patent Office
Prior art keywords
queues
search order
transmission
packets
order table
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP01109085A
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German (de)
English (en)
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EP1146767A3 (fr
Inventor
Wai-Chung Chan
Chi-Jiun Su
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Hughes Network Systems LLC
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Hughes Electronics Corp
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/04—Selecting arrangements for multiplex systems for time-division multiplexing
    • H04Q11/0428—Integrated services digital network, i.e. systems for transmission of different types of digitised signals, e.g. speech, data, telecentral, television signals
    • H04Q11/0478—Provisions for broadband connections
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L12/00—Data switching networks
    • H04L12/54—Store-and-forward switching systems 
    • H04L12/56—Packet switching systems
    • H04L12/5601—Transfer mode dependent, e.g. ATM
    • H04L2012/5603—Access techniques
    • H04L2012/5604—Medium of transmission, e.g. fibre, cable, radio
    • H04L2012/5608—Satellite
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L12/00—Data switching networks
    • H04L12/54—Store-and-forward switching systems 
    • H04L12/56—Packet switching systems
    • H04L12/5601—Transfer mode dependent, e.g. ATM
    • H04L2012/5638—Services, e.g. multimedia, GOS, QOS
    • H04L2012/5646—Cell characteristics, e.g. loss, delay, jitter, sequence integrity
    • H04L2012/5649—Cell delay or jitter
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L12/00—Data switching networks
    • H04L12/54—Store-and-forward switching systems 
    • H04L12/56—Packet switching systems
    • H04L12/5601—Transfer mode dependent, e.g. ATM
    • H04L2012/5678—Traffic aspects, e.g. arbitration, load balancing, smoothing, buffer management
    • H04L2012/5681—Buffer or queue management

Definitions

  • the present invention relates generally to communication systems. and is more particularly related to providing fairness in the servicing of queues in a switching communication system.
  • Network latency is attributable, in part, to queueing delays.
  • queueing delays are difficult to predict or bound, in part, because of the correlation between the output traffic and the input traffic, which can exhibit a wide variety of behaviors (e.g., stochastic, deterministic, or a combination thereof).
  • applications cannot efficiently determine a timeout period for retransmitting lost or greatly delayed packets.
  • the end users are unable to receive a guarantee on their application response times.
  • the delays are not uniform across the users; that is, some users, by virtue of the relative position of the queues that store their traffic, may experience significantly more delay than other users. Thus, the queues are not serviced fairly during network congestion, for example.
  • a method for minimizing queueing delay of packets in a communication system.
  • the method includes retrieving a search order table that has a plurality of table entries corresponding to M queues that selectively store the packets.
  • the table entries store values that correspond to the relative positions of the M queues and that are selected based upon a transmission constraint of the communication system.
  • the method also includes scheduling transmission of the packets that are stored in the M queues based upon the search order table. Under this approach, the output delay is minimized, and the upper bound is known.
  • a communication system for minimizing queueing delay of packets comprises M queues that are configured to store the packets.
  • a memory stores a search order table that has a plurality of table entries corresponding to the M queues.
  • the table entries store values that correspond to relative positions of the M queues and that are selected based upon a transmission constraint of the communication system.
  • a scheduler is coupled to the memory and is configured to schedule transmission of the packets stored in the M queues based upon the search order table. The above arrangement controls queueing delays and provides fair servicing of the queues.
  • a switching device comprises a plurality of input ports. Each of the input ports is configured to receive a plurality of packets. A plurality of queues are configured to store the plurality of packets. A memory stores a search order table that has a plurality of table entries. The table entries store values that correspond to relative positions of the plurality of queues and that are selected based upon a transmission constraint, wherein the number of queues is M . A scheduler is coupled to the memory and configured to schedule transmission of the packets stored in the queues based upon the search order table. A plurality of output ports are configured to transmit the stored packets in the plurality of queues based upon the transmission constraint, wherein the stored packets in N number of M queues are selected for transmission. Under this arrangement, the worst-case queueing delay can be minimized, and the upper -bound can be computed independent of the traffic load and the traffic characteristics.
  • a computer-readable medium carrying one or more sequences of one or more instructions for minimizing queueing delay of packets in a communication system.
  • the one or more sequences of one or more instructions include instructions which, when executed by one or more processors, cause the one or more processors to perform the step of retrieving a search order table that has a plurality of table entries corresponding to M queues that selectively store the packets.
  • the table entries store values that correspond to relative positions of the M queues and that are selected based upon a transmission constraint of the communication system.
  • Another step includes scheduling transmission of the packets stored in the M queues based upon the search order table. This approach advantageously provides fair servicing of queues.
  • the present invention minimizes queueing delay of packets, in which multiple queues are configured to store packets for transmission.
  • a memory stores a search order table that has table entries corresponding to the queues. Specifically, the table entries store values that correspond to the relative positions of the queues and that are selected based upon a transmission constraint of the communication system.
  • a scheduler schedules transmission of the packets that are stored in the queues based upon the search order table.
  • a class of search order tables are generated that satisfy multiple transmission constraints. In addition, by repeating table entries in the search order table, unbalanced traffic loads can be accommodated.
  • This queueing mechanism has applicability to a packet-switched satellite communication system with an on-board switch; the switch has transmission constraints such that the transmitters of the satellite at the output port of the switch cannot simultaneously transmit to downlink cells that contain station terminals that are within an interfering distance from one another.
  • FIG. 1 shows a functional block diagram of a switch with a queueing mechanism, according to an embodiment of the present invention.
  • Switch 100 includes multiple input ports 101 that receive incoming traffic from a source node (not shown)
  • a packet buffer 107 stores packets from input ports 101, in which the stored packets in packet buffer 107 are transmitted to a scheduler 109.
  • the scheduler 109 communicates with a constraint check logic 111 to determine whether the stored packets conform with established transmission constraints.
  • Scheduler 109 examines the destination addresses of the packets that are stored in packet buffer 107 using a round-robin scheme and forwards such packets to an appropriate output port among the various output ports 113.
  • the scheduler 109 schedules the packet transmissions using a search order table that is stored in a memory 115.
  • the search order table effectively orders the queues within the packet buffer 107 according to various transmission constraints to maximize system throughput.
  • the search order table that is stored in memory 115 in an exemplary embodiment, can be retrieved from a class of search order tables that are stored in a database 117, which can be either locally located within the switch 100 or externally located from the switch 100.
  • FIG. 2 shows a communication network that utilizes the switch of Figure 1.
  • a communication network 200 includes multiple source stations 201 that generate traffic to node 203, which can be any networking equipment that transfers data.
  • node 203 is an internetworking device, such as a router; alternatively, node 203 may be any type of gateway in a land-based or satellite-based communication system.
  • Node 203 is connected to an input port ( Figure 1) of switch 100.
  • the output ports ( Figure 1) of switch 100 connects to multiple nodes 205, which can be the same networking component as that of node 203.
  • each of the nodes 205 can potentially communicate with numerous destination stations 207 within region 209 (e.g., sub-network). For example, if nodes 205 are routers, the routers would have multiple output ports designated for region 209.
  • the conventional communication network exhibits performance characteristics that are dictated largely by the hardware limitations of switch 100.
  • the throughput of the network 200 depend on such parameters as buffer size and processing capability of switch 100.
  • the communication network 200 possess network bottlenecks at points in the network other than the switch 100. For example, assuming that for security reasons, destination stations 207 within region 209 cannot simultaneously receive packets, consequently switch 100 may need to buffer some of the packets until the first set of packets are delivered to the particular destination stations 207. Thereafter, the buffered packets within switch 100 can be delivered to the destination stations 207 within region 209. From this example, it is clear that the buffering of the packets within switch 100 can result in system performance that does not depend on the hardware capabilities of switch 100, but instead on the network constraints associated with region 209.
  • the above scenario is characteristic of a satellite communication system.
  • the operation of the queueing mechanism according to an embodiment of the present invention is discussed with respect to a satellite communication system with transmission constraints to the downlink cells.
  • the approach has applicability to many other switching systems, as recognized by one of ordinary skill in the art.
  • the switching systems may include an ATM (Asynchronous Transfer Mode)/SONET (Synchronous Optical Network) network, a Gigabit Ethernet network, and voice network.
  • the end stations of these systems are referred to as destination sites. Accordingly, the destination sites in a satellite system would be downlink cells.
  • FIG. 3 shows a satellite communication system with an on-board switch, in accordance with an embodiment of the present invention.
  • the communication system 300 is a packet-switched satellite communication system, which includes a satellite payload 301 that has a switch 302.
  • the switch 302 is connected to multiple transmitters 303; that is, N transmitters.
  • Switch 302 includes a downlink scheduler 309, and a constraint check logic 311.
  • the downlink scheduler 309, and constraint check logic 311 may be implemented via software, hardware (e.g., general processor, an Application Specific Integrated Circuit (ASIC), etc.), firmware or a combination thereof.
  • ASIC Application Specific Integrated Circuit
  • Each output port (not shown) is associated with a physical location (or destination) on the footprint of the satellite which has its own required transmit power and azimuth and elevation angles. Different destinations are said to be interfering with one another if their azimuth and elevation angles are within a predetermined system interference limit. In other non-satellite applications, "interference" between different destinations or output ports may be due to the sharing of a common resource, such as a transmitter or a bus.
  • station terminals (ST) (not shown) originate traffic from a coverage area 315.
  • the generated traffic from the STs is transferred through switch 302 and terminate at destination STs (not shown) within coverage area 317. It should be noted that the destination STs can be within the same coverage area 315 as the originating STs.
  • a connection that is established between a source ST and a destination ST is controlled by a network operation center (NOC).
  • NOC network operation center
  • the NOC (not shown), which is based on the ground, provides management functions for the system 300.
  • the database 117 ( Figure 1) that stores the search order tables is maintained in the NOC.
  • An ST needs to obtain authorization from the NOC before making a request to the satellite payload 301.
  • the ST is likely to receive a rate allocation from the satellite payload 301 because the NOC keeps track of the total uplink (and downlink) bandwidth available for connections and will block a connection request if there is insufficient satellite capacity available to satisfy the request.
  • the scheduler logic 309 utilizes a class of search order tables, which are stored in the memory 115 ( Figure 1) of the switch 302.
  • the switch 302 searches for qualified transmissions such that the resultant worst-case queueing delays of all transmissions can be minimized and the upperbound can be computed, independent of the pattern and variation of the traffic arriving at the switch 302.
  • the class of search order tables is constructed by taking into account both interference between destinations and transmitted power to destinations.
  • the system 300 operates in the 29.5 - 30.0 GHz Earth to Space direction and operates in the 19.7 - 20.2 GHz Space to Earth direction (i.e., "A" band), in accordance with one embodiment.
  • A Space to Earth direction
  • the 500 MHz is divided into 8 sub-bands each 62.5 MHz wide.
  • the frequency band that is used to carry traffic from STs to a satellite's payload 301 within each cell (uplink beam) encompasses one or more 62.5 MHz sub-bands.
  • Each sub-band is further divided into a number of channels that operate at three different bit rates.
  • the 512 kbps channels have two operating modes, a normal mode, and a fall-back mode, which can be used during rain fades to provide additional link margin.
  • the uplink frame structure in an exemplary embodiment, during normal operation is a 96ms frame with 32 slots of 3ms each, supporting uplink channel rates 512 kbps, 2 Mbps, and 16 Mbps.
  • the number of packets within a slot varies by channel rate.
  • Each time slot is sized to match one uplink-transmitted burst.
  • Each burst has a header and a body.
  • the header contains synchronizing information so the satellite demodulator (not shown) can recover the burst.
  • the body contains two, eight or sixteen data packets, corresponding to transmission rates of 512 kbps, 2 Mbps, and 16 Mbps, respectively.
  • an ST within coverage area 315 transmits at a maximum rate of 512 kbps. 2 Mbps, and 16 Mbps. However, in the presence of fading, a 2 Mbps ST may switch down to a 512 kbps channel. Similarly, an ST transmitting at 512 kbps may switch down to a 128 kbps fall-back channel.
  • the 128 kbps mode (or fall-back mode) uses eight slots per frame instead of 32 slots per frame, with two data packets per burst. All transmission rates use Offset QPSK modulation; filtering is 25 percent raised root cosine.
  • the 128 kbps transmission rate is intended as a mechanism to reduce the uplink rate during poor propagation conditions to improve link availability.
  • the satellite may have 224 active uplink demodulators (not shown).
  • Each uplink demodulator supports the equivalent of three 16 Mbps channels.
  • Each 16 Mbps channel can be configured as a single 16 Mbps channel or eight 2 Mbps channels.
  • each of the eight channels can be utilized as a single 2 Mbps channel or four 512 kbps channels.
  • the capacity of the satellite is 21,504 channels if all the channels are configured as 512 kbps.
  • the communication system 300 supports two types of uplink channels: (1) contention channels, and (2) data channels.
  • An uplink channel can be configured as either a contention channel or a data channel at any given time, but not both simultaneously.
  • Contention channels are used by an ST for bandwidth allocation requests to the satellite payload 301.
  • Bandwidth allocations are made periodically by the satellite payload 301.
  • the satellite payload 301 transfers any totally unallocated data channels to the contention channels.
  • Allocations are packed into a downlink multicast to all ST in each uplink beam. This multicast also provides notification of any additional contention channels, above the already configured contention channels, which are available to the ST in the beam.
  • the service areas 315 and 317 are covered by a set of polygons that are fixed on the surface of the earth.
  • Downlink polygons called microcells
  • microcells are hexagonal in shape as viewed from the spacecraft, with seven microcells clustered together to form an uplink polygon, called a cell.
  • the term microcell is used synonymously with the term downlink cell.
  • the satellite generates a set of uplink circular beams that each encloses a cell. It also generates a set of downlink beams that each encloses a microcell.
  • Polarization is employed to maximize the system capacity.
  • the polarization is fixed, as are the satellite beams that serve the cells. Adjacent cells or cells that are separated by less than one cell diameter of the same polarization must split the spectrum; that is, such cells cannot use the same frequencies. However, adjacent cells on opposite polarization can use the same frequencies.
  • the downlink beam operates on two polarizations simultaneously so that the frequency reuse ratio is 2:1.
  • a total of 24 transmitters, 12 on RHC (Right-Hand Circular) polarization and twelve on the LHC (Left-Hand Circular) polarization serve the downlink cells.
  • the transmitters serve all microcells by time hopping from microcell to microcell. With 24 transmitters, the theoretical frequency reuse ratio is 24:1.
  • Up to 12 downlink spot beams can be transmitted simultaneously on each polarization subject to minimum microcell separation distance limitations. Beams on the same polarization must be sufficiently separated spatially to avoid unacceptable co-channel interference. Another co-polarized beam is not allowed to transmit to another microcell within an ellipse or else excessive interference may occur.
  • the "keep-out" areas apply separately and independently for the two polarizations; the link budgets account for any cross-polarization interference that may occur.
  • the downlink scheduler 309 selects up to N bursts of packets from M virtual queues of the packet buffer 307 to transmit through N transmitters, based on the scheduling algorithm and transmission constraint checks.
  • the scheduling algorithm in an exemplary embodiment, is a round-robin scheme. Because the downlink scheduler 309 may not be able to find N bursts to transmit most of the time due to transmission constraints, downlink transmission capacity is greatly limited by transmission constraints.
  • the downlink congestion in communication system 300 occurs when the amount of traffic admitted to the switch 302 exceeds the capacity of the downlink.
  • the satellite payload 301 if the satellite payload 301made uplink allocations simply based on the availability of uplink slots, the satellite payload 301 would sometimes admit more traffic to a particular downlink cell (i.e., destination site) or cluster of mutually-interfering microcells than the downlink can carry. Consequently, the data packets for these areas would completely fill the packet buffer 307 in the payload's switch 302, resulting in dropped packets. Therefore, the availability of both uplink slots and downlink bandwidth factor into bandwidth allocations that is performed by the satellite payload 301.
  • the main transmission constraint in communication system 300 is the interference constraint: that is, two simultaneous downlink transmissions cannot be performed if they are directed at downlink cells which are within a system limit interference distance.
  • the satellite can simultaneously transmit packets to these downlink cells A, B and C.
  • simultaneous transmission cannot be directed to downlink cells D and A, downlink cells D and B, downlink cells E and B and downlink cells E and C since they are within the system limit interference distance. That is, these downlink cells are in the same circle.
  • FIG. 4 shows a diagram of an interference region of a target downlink cell defined according to an embodiment of the present invention.
  • An interference region 400 includes a target downlink cell, which is surrounded by numerous downlink cells 401.
  • Downlink cells 401 are clustered around target downlink cell Y within a radius that is determined by an angle x from a point of view of a satellite.
  • the angle x can be set to any degree, depending on the coverage area and network application.
  • the transmission constraint of the system 300 dictates that simultaneous transmissions cannot be directed to any two downlink cells, which are within x degrees from each other, from the point of view of the satellite.
  • the satellite transmits to a target downlink cell
  • the satellite cannot transmit, at the same time, to any downlink cell around the target downlink cell that is within x degrees from the target downlink cell.
  • the group of all the downlink cells, which are within x degrees from a target downlink cell is defined as the interference region of the target downlink cell.
  • the interference region 400 of a downlink cell can be obtained by comparing angles between any two downlink cells from the point of view of the satellite.
  • the destinations for the packets that stored within the queues 307 are power constrained, such that two interference regions 503 and 505 are considered. That is, any two destinations associated with queues 1 to 7 are interfering with one another, and any two destinations associated with queues 8 and 14 also interfere with each another. Accordingly, any one destination 1 to 7 does not interfere with any destination from 8 to 14.
  • the switch 302 can transmit up to 2 bursts in each downlink time slot, and that there are initially many bursts in each of the 14 destination queues such that they are never empty.
  • the initial seed is destination 1. Accordingly, one burst is transmitted from queue 1.
  • bursts from destinations 2 through 7 cannot be selected because they interfere with the burst from destination 1. Consequently, the burst from queue 8 is transmitted.
  • the initial seed is destination 2, in which one burst will be transmitted from destinations 2 and 8. It is noted that bursts from destinations 3 to 7 cannot be selected because they interfere with the burst from destination 2.
  • the initial seed is destination 7; accordingly, one burst will be transmitted from destinations 7 and 8 because queues 7 and 8 belong to different interference regions 503 and 505.
  • the initial seed is destination 8. Because destinations 9 to 14 are a part of the same interference region 505 as that of queue 8, the next suitable queue is queue 1.
  • the initial seed is destination 9, in which one burst will be transmitted from destinations 9 and 1.
  • the initial seed is destination 10, such that one burst will be transmitted from destinations 10 and 1.
  • queueing delays for queues 9-14 can be less if queue 8 has no packets to send. For instance, if queues 8 to 13 are empty, then the queueing delay for bursts from destination 14 will be small. Thus, the behavior of the queueing delay depends heavily on the traffic load of each of the queues and can fluctuate widely as the traffic load changes. By constructing a search order table that takes into account the transmission constraints, the worst-case queueing delay can be minimized, according to the process of Figure 6.
  • the constraint check logic 311 ( Figure 3) checks whether the two transmission constraints are satisfied.
  • the transmission constraints include the following: (1) the destinations, to which the stored bursts are to be sent, do not interfere with one another, and (2) the total power required by all the transmissions is smaller than the peak available power. If the particular queue meets the two transmission constraints, the scheduler 309 includes the queue on the transmission list, per step 609. Subsequently, it is determined, as in step 611, whether there are N such queues in the transmission list or whether all M queues have been examined; if not, the next queue in the search order table is to be examined (step 613).
  • step 613 performed in which the next queue is examined.
  • the search process continues until N qualified queues have been found or until all the entries in the search order table have been checked.
  • one burst will be transmitted from each of the queues in the transmission list.
  • Sufficient time is allocated to one downlink time slot such that the search process and all the eligible transmissions can be completed. Under this approach, the worst-case queueing delay is minimized, in addition to achieving a more equitable scheme of scheduling.
  • the potentially long queueing delay that is suffered by delay-sensitive bursts may render the transmissions useless. As a result, the useful capacity of the system 300 will be reduced, and more importantly, the system 300 may not be able to function properly.
  • Figure 7A shows a diagram of a search order table, in accordance with an embodiment of the present invention.
  • the process of Figure 6 results in a search order table 701 that the resultant worst-case queueing delays of all transmissions of the bursts within the corresponding queues can be upper-bounded by some pre-determined values, independent of the pattern and variation of the traffic arriving at the switch 302.
  • the search order table 701 dictates that the queues are to be served in the following order: queue 1, queue 8, queue 2, queue 9, queue 3, queue 10, queue 4, queue 11, queue 5, queue 12, queue 6, queue 13, queue 7, and queue 14.
  • bursts from queues 1 and 8 are transmitted.
  • t 2 bursts from queues 8 and 2 are sent.
  • Figure 7B shows an exemplary search order table 703, which accommodates uneven traffic loading of the queues, according to an embodiment of the present invention. If one queue 307 ( Figure 3) is experiencing higher traffic over the other queues, additional table entries corresponding to the queue can be stored. By allowing destinations with high traffic loads to be repeated in the search order table, the switch 302 can schedule more transmissions from those destinations so that they are provided with a larger share of the system capacity. In this example, both queues 7 and 14 experience twice as much traffic as the other queues 1-6, and 8-13; consequently, each of the queues 7 and 14 has two entries in the search order table 703.
  • queue 7 is served; likewise, during time slots t 7 , t 8 , t 15 , and t 16 , queue 14 is served.
  • queues 7 and 14 are given four opportunities to transmit packets during a single period, while the other queues 1-6, and 8-13have two opportunities.
  • search order tables 701 and 703 As a further refinement in search order tables 701 and 703, other constraints can be imposed to construct a search order table that better reflects the particular communication system that employs the scheduler 309.
  • FIGS 8A and 8B show exemplary search order tables with two different starting points, in accordance with an embodiment of the present invention. Assuming that each queue (or destination) can appear in the search order table only once, the search order table will have M entries.
  • the switch 302 is capable of transmitting up to N non-interfering bursts in each downlink time slot.
  • the constructed search order table seeks to guarantee a reasonably small upper bound for the queueing delay for all the queues, independent of the traffic load.
  • the class of search order tables is characterized by the parameters ( K , L ) and exhibit two properties (P1 and P2).
  • the first desirable property (referred to as "P1") of the search order tables is that any K consecutive entries in the table do not interfere with one another. Interfering destinations are placed as far apart as possible in the search order table 801. Thus, K should be as large as possible; at a minimum, K ⁇ N .
  • P1 the first desirable property of the search order tables is that any K consecutive entries in the table do not interfere with one another. Interfering destinations are placed as far apart as possible in the search order table 801.
  • K should be as large as possible; at a minimum, K ⁇ N .
  • P1 By imposing property P1, it is guaranteed that each queue (or destination) will be selected for transmission at least min (K, N ) times in one period, provided that its queue is not empty and only the interference constraint is considered.
  • a period is defined as the number of downlink time slots that is required for the initial seed of the scheduler 309 to go through each of the entry in the search order table once. For each queue or destination in the search order table in which the initial seed is the particular queue or is min( K , N )-1 entries preceding the particular queue, the particular queue or destination will be selected for transmission (i.e., added to the transmission list)
  • each destination is associated with a required transmit power, and the total allowable transmit power in a downlink time slot is limited. This assumption is particularly relevant to satellite communications systems.
  • each destination requires either a low-powered or high-powered transmission.
  • a low-powered and a high-powered transmission correspond to a point-to-point and a cellcast destination, respectively.
  • destinations that require high-powered transmissions should be placed as far as possible from each other.
  • the second desirable property (referred to as "P2") of the search order tables is that any L consecutive entries in the table can have only one or zero destination that requires a high-powered transmission.
  • the largest possible value of L is desirable.
  • P2 it is guaranteed that a large number of consecutive entries are selected for transmission in each downlink time slot in which at most a single high-powered transmission is required. Hence, the worst-case queueing delay can be kept to a low value.
  • Search order table 801 has K equal to 12 and L equal to 6.
  • table 801 has two fields: a Position field 801a and a Type field 801b.
  • the Position field 801a stores values corresponding to the M queues that are to be positioned for transmission according to the placement in the table 801.
  • the Type fields 801b specifies the power requirement for the particular queue or destination.
  • PTP stands for point-to-point (or low-powered)
  • CC stands for cellcast (or high-powered) destinations.
  • the queueing delay bound for point-to-point (or low-powered) transmission is considered, with particular focus on the point-to-point entry at position x .
  • the entry at position x will be selected for transmission if any of the following is true: (1) the initial seed of the scheduler is at position x ; (2) the initial seed of the scheduler is at position ( x -1), ( x -2), ..., or ( x -6), and the sum of power ( p + q ), ( p +2 q ), ..., or ( p+ 6 q ), respectively, is less than or equal to Pmax , where Pmax is the maximum available transmit power, p is the transmit power of a high-powered (or cellcast) transmission, and q ( p ⁇ q ) is the transmit power of a low-powered (or point-to-point) transmission; and (3) the initial seed of the scheduler is at position (x-7), ( x -8), ..., or
  • QL represents the queue length.
  • Size It should be noted that when each destination appears only once in the search order table, Size and M are the same.
  • the entry at position y will be selected for transmission if any of the following is true: (1) the initial seed of the scheduler is at position y ; (2) the initial seed of the scheduler is at position ( y -1), ( y -2), ..., or ( y -5), and the sum of power ( p+q ), ( p+ 2 q ), ..., or ( p +5 q ), respectively, is less than or equal to Pmax ; and (3) the initial seed of the scheduler is at position ( y -6), ( y -7), ..., or ( y -11) and the sum of power (2 p+ 5 q ), (2 p +6 q ), ..., or (2 p+ 10 q ), respectively, is less than or equal to Pmax .
  • the cellcast entry at position y will be transmitted ( m +2) times in one period. If that entry appears R times in the table, then the queueing delay bound for cellcast transmission is as follows:
  • Figure 9 shows a chart of the queueing delay bounds, in accordance with an embodiment of the present invention.
  • Equation (Eqs. 2-4) factor in the queue length QL of each queue.
  • n is the largest integer in ⁇ 6,7,...,10 ⁇ such that 2 p + nq ⁇ P max .
  • m is the largest integer in ⁇ 5,6,...,10 ⁇ such that 2 p + mq ⁇ P max .
  • a request packet 1000 includes the following fields: a destination address field 1001; an uplink rate field 1003; a request type field 1005; a rate request field 1007; a destination downlink field 1009; and a request priority field 1011.
  • the destination address field 1001 specifies the requesting ST's destination address.
  • the uplink rate field 1003 indicates the uplink rate; e.g., 128kbps, 512kbps, 2 Mbps, or 16 Mbps.
  • the request type field 1005 indicates whether the request is a rate or volume allocation.
  • the rate request field 1007 permits the ST to specify the requested rate or number of time slots requested.
  • the destination downlink field 1009 specifies the downlink cell where the packets in the requested slots are to be sent.
  • the request priority field 1011 allows the ST to indicate whether the request is a low or high priority. The satellite processes low priority requests, for example, only if there are slots remaining after all high priority requests have been filled.
  • the allocation packet 1020 contains individual rate and volume allocations with the following information: an ST source address field 1021, a rate/number of slots field 1023, and a last allocation of request field 1025 (meaningful for volume requests only).
  • the address field 1021 stores the address of the requesting ST.
  • the rate/number of slots field 1023 indicates the requested rate (i.e., number of slots in the frame for a given channel).
  • the field 1025 pertains only to volume requests, and specifies whether this is the last allocation.
  • the acknowledgement packet 1040 contains individual acknowledgements or denials with the following fields: an ST source address field 1041, a request ID 1043, and a type field 1045.
  • the ST source address field 1041 is the same as field 1021 of the allocation packet.
  • the request ID (identification) field 1043 indicates a particular request for the ST, so that in a volume request, the potentially numerous follow-up requests can be properly managed.
  • the type field 1045 specifies that the BCP 305 is denying the request.
  • FIG. 11 illustrates a computer system 1101 upon which an embodiment according to the present invention may be implemented to minimize the queueing delays.
  • Computer system 1101 includes a bus 1103 or other communication mechanism for communicating information, and a processor 1105 coupled with bus 1103 for processing the information.
  • Computer system 1101 also includes a main memory 1107, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1103 for storing information and instructions to be executed by processor 1105.
  • main memory 1107 may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 1105.
  • Computer system 1101 further includes a read only memory (ROM) 1109 or other static storage device coupled to bus 1103 for storing static information and instructions for processor 1105.
  • ROM read only memory
  • a storage device 1111 such as a magnetic disk or optical disk, is provided and coupled to bus 1103 for storing information and instructions.
  • Computer system 1101 may be coupled via bus 1103 to a display 1113, such as a cathode ray tube (CRT), for displaying information to a computer user.
  • a display 1113 such as a cathode ray tube (CRT)
  • An input device 1115 is coupled to bus 1103 for communicating information and command selections to processor 1105.
  • cursor control 1117 is Another type of user input device, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 1105 and for controlling cursor movement on display 1113.
  • Non-volatile media includes, for example, optical or magnetic disks, such as storage device 1111.
  • Volatile media includes dynamic memory, such as main memory 1107.
  • Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1103. Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communication.
  • Computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.
  • Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 1105 for execution.
  • the instructions may initially be carried on a magnetic disk of a remote computer.
  • the remote computer can load the instructions relating to the notification services to control call processing remotely into its dynamic memory and send the instructions over a telephone line using a modem.
  • a modem local to computer system 1101 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal.
  • An infrared detector coupled to bus 1103 can receive the data carried in the infrared signal and place the data on bus 1103.
  • Bus 1103 carries the data to main memory 1107, from which processor 1105 retrieves and executes the instructions.
  • the instructions received by main memory 1107 may optionally be stored on storage device 1111 either before or after execution by processor 1105.
  • Computer system 1101 also includes a communication interface 1119 coupled to bus 1103.
  • Communication interface 1119 provides a two-way data communication coupling to a network link 1121 that is connected to a local network 1123.
  • communication interface 1119 may be a network interface card to attach to any packet switched local area network (LAN).
  • communication interface 1119 may be an asymmetrical digital subscriber line (ADSL) card, an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line.
  • Wireless links may also be implemented.
  • communication interface 1119 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.
  • a class of search order tables are generated that satisfy two properties: any K consecutive entries in the table do not interfere with each other, and any L consecutive entries in the table can have only one or zero destination that requires a high-powered transmission.
  • the resultant worst-case queueing delay of a packet switch communication system can be upper-bounded by some reasonably small values.
  • by repeating busy destinations in the search order table while maintaining the two properties it is possible to increase the capacity for the busy destinations.

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MXPA01003811A (es) 2004-11-10
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US7370116B2 (en) 2008-05-06

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